Species mixtures are of great interest to promote low-input agricultural practices while maintaining agricultural production. For example, winter oilseed rape sown with a frost sensitive legume reduces the use of nitrogen fertilizer and herbicide while maintaining yield at levels equivalent to pure cropping (Lorin et al., 2016). A disadvantage of such mixtures is that the legume might compete with the cash crop for resources and reduce yield. To optimize management of these mixtures, it is necessary to better understand the ecophysiological processes that drive the sharing of resources between plants (e.g. carbon and nitrogen). Mechanistic functional-structural modeling (FSM) is particularly suitable for such studies as it allows the analysis of the processes underlying plant-plant interactions and integrates both the architectural growth processes of the plant and its functioning (e.g. acquisition/allocation of resources) (Louarn and Song, 2020). For example, the model Virtual Grassland (Louarn et al., 2014) simulates competition for C, N and water between several herbaceous and legume species. However, no FSM for mixtures of annual crops with frost sensitive legume that considers the feedback of the carbon/nitrogen functioning in both shoots and roots has been developed so far. Such a modeling approach is shown in this study, with the aim of filling knowledge gaps on the role of carbon-nitrogen interactions in plastic responses of plants in mixtures, by taking the case study of winter oilseed rape sown with frost sensitive fababean at vegetative stage.